If your Google account is already added to your Mac System Preferences with contacts enabled, you need to disable the contacts switch for the best results. Read more about it at https://playaapps.zendesk.com/hc/en-us/articles/226488587.
Make sure that you've given this app access to your Mac Contacts. Otherwise, this app won't be able to sync your contacts. To make sure that this app has access to your contacts, go to your Mac System Preferences > Security & Privacy > Privacy > Contacts, and make sure that this app is enabled.
Before syncing with Gmail for the first time, log into Gmail.com and make sure that you don't have any duplicate contacts. Even though it may appear as if you don't have duplicates, Gmail may be masking the problem. Read more about it at https://playaapps.zendesk.com/hc/en-us/articles/226488607.
If this is your first time syncing, and you can treat either your Gmail or Mac contacts as your "master" list, I would suggest using either the "Gmail Contacts -> Mac" sync mode or the "Mac Contacts -> Gmail" sync mode. Otherwise, perform a 2-way sync.
After your first sync, I would generally recommend using a 2-way sync.